During the operation of steam turbines in a boiler fired power, often there is a need to run turbines at low load levels for extended periods of times. Conventional steam power plants are designed to operate at rated load, and thus operation at low load level results in a decrease in energy utilization and efficiency.
When the load of a steam turbine is reduced, pressure in the reheat system drops in direct proportion to the steam flow. Reheat pressure is a sensitive parameter in a power plant. This is because in most steam power plants the highest feedwater heater is connected to the cold reheat system. The cold reheat pressure is directly related to the feedwater temperature at the boiler inlet. Thus, when the cold reheat pressure is reduced, the feedwater temperature at the boiler inlet is also reduced. Further, with a reduced reheat pressure, the temperature at the outlet of the hot reheat system will drop, resulting into reduced cycle efficiency.
At low loads, it is therefore advantageous to maintain the reheat pressure at high levels in order to avoid imposing temperature related stresses on boiler and turbine parts. Further, it is very advantageous to maintain the reheat pressure at high levels because with elevated back pressure, the duty of the reheat system is reduced while maintaining a generally constant steam flow rate. As a result, the temperature levels at steam turbine exhaust rises and also the outlet of the hot reheat system will rise correspondingly.
One system for maintaining temperatures at low load, includes extracting steam from a steam generator into a heat reservoir, for use in other systems or process, in order to reduce the mass flow of steam through the superheater system, so that the live steam temperature is increased. This solution, however, requires a conduit connection point on the steam generator to accommodate the extracted steam, and further does not provide an increase in pressure of the reheat system.
Other systems increase pressure at the inlet of the highest top heater of the water
steam cycle by shifting the extraction point to a higher pressure level, which will only be possible when this high pressure extraction is available. However, retrofitting a power plant to this solution requires an additional extraction point in the system, which is an expensive solution.
None of the existing solutions provide an increase in extraction pressure at the highest top heater, while maintaining the same number of high pressure extraction points on the water-steam cycle.
Therefore, an object of the present disclosure is to provide a thermal power plant, steam turbine, and a control method for a partial load operation that maintains or increase back pressure at low load, minimizes temperature variation, without requiring additional high pressure extraction points.